The broad objective of this study is to develop new approaches to the treatment of human allergic diseases through a more precise understanding of the structure and function of the mast cell. The basis for the allergic reaction is the sensitization of mast cells by reaginic (IgE) antibodies and the release of biologically active mediators upon reaction of specific allergen with sensitized cells. A transplantable mouse mast cell tumor, the ascitic form of the Furth mastocytoma, retains the ability to bind mouse reaginic antibodies and it contains active histidine decarboxylase, i.e., histamine is synthesized. The avialability of large numbers of mastocytoma cells provides a means to isolate and characterize specific membrane receptors for IgE antibody and to study the effect of the allergic reaction on histidine decarboxylase activity. The receptor will be detected by its ability to absorb IgE antibody from mouse serum; residual antibody activity will be assayed by in vivo (passive cutaneous anaphylaxis) and in vitro (passive sensitization of normal mouse peritoneal mast cells) methods. Histidine decarboxylase will be assayed with an enzymic-isotopic method which measures the amount of histamine formed. Finding a way to interfere with the function of these mast cell components may lead to new modes of therapy for human allergic diseases.